1. Field of the Invention
This invention relates generally to tension leg platforms used in the offshore oil and gas production industry and specifically to a method and system for ballasting and de-ballasting a tension leg platform for towing, installation (lock-off to tendons) and use during in-service operation of the platform.
2. Description of the Prior Art
Tension leg platforms (TLP) are generally used offshore in deep water for the production of hydrocarbons. A typical TLP has a horizontal pontoon hull structure and vertical columns supporting a platform. The hull structure provides buoyancy to the columns and platform. The TLP is anchored by tendons to pilings in the ocean floor, and it is held stationary by buoyancy-induced tension in the tendons.
The hull is generally divided into several watertight compartments in order to meet stability requirements during installation ballasting. TLPs are de-ballasted during installation to tension the tendons to maintain the platform within design limits at all times. The de-ballasting operation is preferably rapid to minimize the time during which the resonant frequency of the TLP equals the natural period of the surrounding water. In order to rapidly de-ballast, TLPs are generally equipped with one or more pump rooms containing high-capacity pumps. However, once installation is complete, only minor in-service trim adjustments are made, so the pumps are no longer subjected to high-capacity requirements.
3. Identification of Features Provided by Some Embodiments of the Invention
A primary object of the invention is to minimize the capital cost of the installed high capacity pumps which pump and distribute ballast water into the hull.
Another object of the system is to provide a method for rapidly achieving installation draft by providing a method of rapid flooding of the ballast tanks.
Another object of the system is to provide a method to limit ratcheting of the slip mechanism when locking off the tendons to the hull.